Polyethylene glycol/polycation block copolymers

ABSTRACT

The invention provides block copolymers formed of poly(ethylene glycol) segments and poly(amino acid derivative) segments having side chains of at least one kind of specific amine residue. The invention also provides polyion complexes of such copolymers with polynucleotides and the like. These block copolymers are useful as carriers for in vivo delivery of active substances such as DNA.

TECHNICAL FIELD

This invention relates to block copolymers having polyethylene glycolstructural portion as a hydrophilic segment and polyamino acidstructural portion having amine residue side chains of variousstructures as a cationic segment; and also to polyion complexes of thecopolymers with nucleic acid or anionic proteins.

BACKGROUND ART

A polyethylene glycol/polycation block copolymer represented bypolyethylene glycol-block-poly(L-lysine) which is a cationic blockcopolymer spontaneously forms a spherical micelle with an anionicmacromolecule, due to the electrostatic interaction acting between thetwo in water as a driving power. This particle has a diameter of severaltens nanometers and a core-shell structure, the core (or inner nucleus)being formed of polyion complex of cation and anion, and the shell (orouter shell) being a polyethylene glycol (which may be hereafterabbreviated as “PEG”) layer. The particle is referred to as polyioncomplex (PIC) micelle (see, e.g., Non-patent Reference 1 which isidentified later, like other references). Thus, PIC micelles can holdanionic macromolecules in inner nuclei and are therefore expected to becapable of avoiding in vivo foreign matter recognizing mechanism due tosuch particle diameter as several tens nanometers and the core-shellstructure. Accordingly, presently their application as a carrier(vector) of DNA which is a natural anionic macromolecule is underinvestigation. Although priority in developing gene vectors using suchcationic block copolymers is thus clear, due to limitations on theirsynthesis and for other reasons, cationic block copolymers which arecurrently investigated do not extend beyond PEG-block-poly(L-lysine),PEG-block-poly(dimethylaminoethyl methacrylate) (see, e.g., PatentReference 1) and PEG-block-polyethylenimine.

These PIC micelles are considerably stable under physiologicalconditions in general, but is actual use their stability underphysiological conditions is occasionally insufficient, as exemplified bydissociation of PIC micelles under dilution after administration byintravenous injection or their interaction with serum proteins. Thisnecessitates modification of properties of PIC micelles so that theywould not dissociate but exist stably for a fixed period, until theyarrive at the intended site with certainty or after their arrival. As ameans to so modify properties of PIC micelles, for example, it has beenproposed to improve stability of PIC micelles by introducingmercaptoalkyl groups into amino groups in a fixed proportion of L-lysineunits in poly(L-lysine) segments in said PEG-block-poly-(L-lysine) toform disulfide bonds between said groups (see, e.g., Patent Reference2).

Also as a new type, a copolymer formed by ester-amide exchange of benzylgroups in PEG-block-poly(β-benzylaspartate) with, for example,N,N-dimethylethylenediamine or the like (see, e.g. Non-patent Reference2).

The function of polycation blocks in the micelles formed of polycationand DNA is mainly to serve as the electrostatic interaction site withthe DNA, while in principle still other functions can be imparted. Asone of such functions, there is proton sponge effect. Proton spongeeffect refers to a phenomenon: when a polyamine of low degree ofprotonation is incorporated in endosomes, it absorbs hydrogen ionssupplied into the endosomes by V-type ATPase one after another toprevent pH drops within the endosomes and in consequence to causeexpansion of the endosomes with water infiltration accompanying rise inosmotic pressure in the endosomes, which eventually leads to destructionof the endosomes. It is expected that transfer of DNA to cytoplasm ispromoted and the gene expression effectiveness is increased by thiseffect. This effect is seen in cations having buffer ability and,therefore, use of cations of low pKa is necessary.

On the other hand, gene expression effectiveness is considered to beaffected also by stability of PIC micelles, condensed state of enclosedDNA and the like, and such factors also are presumed to be dependent onproperties of individual polycation. As aforesaid, however, heretoforethe kinds of studied polycation are limited and there has been noconcept of simultaneous introduction of two or more kinds of polycationsto allot them different functions. Under the circumstances, it was verydifficult to control these factors.

LIST OF CITED REFERENCES

Patent Reference 1: WO98/46655 (cf. pp. 20-21, Examples 10 and 11)

Patent Reference 2: JP2001-146556A

Non-patent Reference 1: Harada and Kataoka, Macromolecules, 1995, 28,5294-5299

Non-patent Reference 2: Polymer Preprints, Japan, Vol. 51, No. 5 (2002)

DISCLOSURE OF THE INVENTION

For example, PEG-block-polycation as described in above Patent Reference2 and Non-patent Reference 2 form polyion complex micelles (PICmicelles) stably enclosing DNA, but provision of PIC micelles exhibitingstill new properties is called for, in consideration of the versatilityin environments of target living bodies to which physiologically activesubstances such as DNA are to be delivered or the optimum release rateof said physiologically active substance from PIC micelles underindividual environment.

We have discovered that a PEG-block-polycation copolymer whosepolycation segment has bulky side chains of low pKa can enclose DNAs insubstantially free state, as contrasted with those copolymers describedin Non-patent Reference 2 in which PIC micelles enclose DNAs inconsiderably condensed state; and also that such DNAs which are enclosedin free state are released at a significantly slower rate than thoseenclosed in condensed state, when, for example, they come to contactwith the target cells to which they are to be delivered, underphysiological conditions.

We have furthermore discovered: when the polycation contains bothprimary amine and secondary or tertiary amine, the primary amine chieflyparticipates in formation of associated particle with DNA, while thesecondary or tertiary amine scarcely participates. Furtherconcentratively pursuing the investigations, we have completed thepresent invention.

Thus, the present invention relates to: (1) a polyethyleneglycol/polycation block copolymer characterized by having segment Aformed of polyethylene glycol or a derivative thereof and segment Bformed of polyamino acid, a derivative thereof or a salt of theforegoing, the segment B containing bulky amines of pKa value not higherthan 7.4 or containing both primary amine and secondary amine, tertiaryamine or quaternary ammonium salt;

as a specific embodiment, (2) the copolymer as described in (1) above,in which the structure of the block copolymer is one represented by ageneral formula (I) or (II)or a salt thereof,

(in which R¹ stands for hydrogen, or a substituted or unsubstituted,straight or branched chain C₁₋₁₂ alkyl, L¹ and L² stand for linkers, R²stands for methylene or ethylene, R³ stands for hydrogen, protectivegroup, hydrophobic group or polymerizable group, R⁴ is either same as R⁵or an initiator residue, R⁵s each independently stands for hydroxyl,oxybenzyl or —NH—(CH₂)_(a)—X group, wherein X each independently standsfor a bulky amine compound residue having a pKa value not higher than7.4 or an amine compound residue containing one, two or more members ofthe group consisting of primary, secondary and tertiary amines andquaternary ammonium salt, or a residue of a compound which is not amine,a is an integer of 1-5, m is an integer of 5-20,000, n is an integer of2-5,000 and x is an integer of 0-5,000, with a proviso that x is notgreater than n);

as a specific embodiment, (3) the copolymer as described in (1) above,in which the structure of the block copolymer is one represented by ageneral formula (III) or (IV) or a salt thereof,

(in which R¹ stands for hydrogen, or a substituted or unsubstituted,straight or branched chain C₁₋₁₂ alkyl, L¹ and L² stand for linkers, R²stands for methylene or ethylene, R³ stands for hydrogen, protectivegroup, hydrophobic group or polymerizable group, R⁴ is either same as R⁵or an initiator residue, R⁵s each independently stands for hydroxyl,oxybenzyl or —NH—(CH₂)_(a)—X group, wherein X each independently standsfor a bulky amine compound residue having a pKa value not higher than7.4 or an amine compound residue containing one, two or more members ofthe group consisting of primary, secondary and tertiary amines andquaternary ammonium salt, or a residue of a compound which is not amine,a is an integer of 1-5, R⁶ each independently stands for hydrogen or aprotective group, wherein the protective group is Z, Boc, acetyl,trifluoroacetyl or the like which are customarily used as protectivegroups of amino, m is an integer of 5-20,000, n is an integer 2-5,000, yis an integer of 0-4,999 and z is an integer of 1-4,999, with theproviso that z is less than n and y+z is not more than n);

as a still more specific embodiment, (4) a copolymer as described in (2)or (3) above, in which R¹ is methyl;

as a more specific embodiment, (5) a copolymer as described in (2) or(3), in which R¹ stands for substituted straight or branched chain C₁₋₁₂alkyl, wherein the substituent is acetalized formyl, cyano, formyl,carboxyl, amino, C₁₋₆ alkoxycarbonyl, C₂₋₇ acylamide, same or differenttri-C₁₋₆ alkylsiloxy, siloxy or silylamino;

as a further specific embodiment, (6) a copolymer as described in anyone of (2)-(5) above, in which L¹ is —(CH₂)_(b)—NH—, b being an integerof 1-5;

as a still more specific embodiment, (7) a copolymer as described in(2); and further in any one of (3)-(5) above, in which L² is—(CH₂)_(c)—CO—, c being an integer of 1-5;

as a still more specific embodiment, (8) a copolymer as described in anyone of (2)-(7) above, in which R² is methylene;

as a still more specific embodiment, (9) a copolymer as described in anyone of (2)-(8) above, in which X is a group represented by the followingGroups A, B, C, D or E,

(in the above formulae, X² stands for hydrogen or C₁₋₆ alkyl, X³ standsfor amino C₁₋₆ alkyl, R⁷ stands for hydrogen or methyl, d stands for aninteger of 1-5, e stands for an integer of 1-5, f stands for an integerof 0-15, R⁸ stands for a protective group, wherein the protective groupis Z, Boc, acetyl, trifluoroacetyl or the like which are customarilyused as protective groups of amino, and g stands for an integer of0-15);

as a still more specific embodiment, (10) a copolymer as described inany one of (2)-(9) above, in which R³ is acetyl, acryloyl ormethacryloyl;

as a still more specific embodiment, (11) a copolymer as described inany one of (2)-(10) above, in which R⁴ is —NH—R⁹, R⁹ standing forunsubstituted or substituted, straight or branched chain C₁₋₂₀ alkyl;

as another embodiment, (12) a polyion complex comprising a copolymer asdescribed in any one of (1)-(11) above and nucleic acid or anionicprotein; and

as a more specific embodiment, (13) a polyion complex as described in(12) above, which is in the form of a polymer micelle carrying nucleicacid or anionic protein in its core portion with the shell portioncomposed mainly of polyethylene glycol segment.

EFFECT OF THE INVENTION

This invention enables to control those factors which affect genetransfer effectivity such as the condensed state of genes in PICmicelles, release rate of the genes from the micelles, proton-spongeeffect, micelle stability and the like. The invention enables provisionof PIC micelles having higher gene transfer effectivity. The inventionalso enables provision of more useful non-viral gene vectors.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a photograph in place of a drawing, showing the result ofelectrophoreses of solutions of MeO-PEG-MOPA and p-DNA blended atvarious N/P ratios (N/P ratio: 0, 10, 5, 4, 3, 2, 1, 0, respectively,from the left). (Here N/P=0 signifies the lane in which p-DNA only ismigrated.)

FIG. 2 is a photograph in place of a drawing showing the result ofelectrophoreses of solutions of MeO-PEG-DET and p-DNA blended at variousN/P ratios (N/P ratio: 10, 5, 3, 2, 1, 0.5, 0.25, 0, respectively, fromthe left). (Here N/P=0 signifies the lane in which p-DNA only ismigrated.)

FIG. 3 is a graph showing particle size distribution of associatedparticles formed of MeO-PEG-MOPA and p-DNA, as measured by means ofdynamic light scatting (DLS).

FIG. 4 is a graph showing the measured result of N/P ratio dependency ofparticle diameter of associated particles formed of MeO-PEG-MOPA andp-DNA.

FIG. 5 is a graph showing particle size distribution of associatedparticles formed of MeO-PEG-DET and p-DNA, as measured by means ofdynamic light scatting (DLS).

FIG. 6 is a graph showing the measured result of N/P ratio dependency ofparticle diameter of associated particles formed of MeO-PEG-DET andp-DNA.

FIG. 7 is a graph showing the zeta potential measurement result ofassociated particles formed of MeO-PEG-MOPA and p-DNA.

FIG. 8 is a graph showing the zeta potential measurement result ofassociated particles formed of MeO-PEG-DET and p-DNA.

FIG. 9 is a graph showing the result of ethidium bromide assay (10 mMTris-HCl buffer, pH=7.4) of associated particles formed of MeO-PEG-MOPAand p-DNA.

FIG. 10 is a graph showing the result of ethidium bromide assay (10 mMTris-HCl buffer+150 mM NaCL, pH=7.4) of associated particles formed ofMeO-PEG-MOPA and p-DNA under physiological saline concentrationcondition.

FIG. 11 is a graph showing the result of ethidium bromide assay (10 mMTris-HCl buffer, pH=7.4) of associated particles formed of MeO-PEG-DETand p-DNA.

FIG. 12 is a graph showing gene transfer effectivity from associatedmicelles formed of MeO-PEG-MOPA and p-DNA (N/P=5) into 293 T-cells (by 4hours' and 24 hours' incubation, respectively).

FIG. 13 is a graph showing gene transfer effectivity from associatedmicelles formed of MeO-PEG-DET, MeO-PEG-DMAPA or MeO-PEG-DAP and p-DNA(N/P=10) into 293 T cells (by 24 hours' incubation).

BEST EMBODIMENTS FOR WORKING THE INVENTION

In the general formulae (I), (II), (III) or (IV), R¹ stands for hydrogenor unsubstituted or substituted straight or branched chain C₁₋₁₂ alkyl.As C₁₋₁₂ alkyl, methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl,tert-butyl, n-pentyl, n-hexyl, decyl, undecyl and the like can be named.When the alkyl groups are substituted, as the substituent acetatalizedformyl, cyano, formyl, carboxyl, amino, C₁₋₆ alkoxycarbonyl, C₂₋₇acylamide, same or different tri-C₁₋₆ alkylsiloxy, siloxy or silylaminocan be named. Where the substituent is acetalized formyl, it can behydrolyzed under mildly acidic conditions to be converted to formyl(—CHO: or aldehyde) which is another substituent. Such formyl, carboxylor amino group can be present, for example, at the shell portion ofpolyion complex micelle of a copolymer following the present inventionand nucleic acid or anionic proteins and can be utilized for covalentlybonding with the micelles antibodies or fragments having the specificbindability thereof (F(ab′)2, F(ab), and the like) and proteins whichare capable of imparting to the micelles other functionality or targetdirectivity, via these groups. PEG segments having such functionalgroups at one end can be conveniently formed by, for example, thepreparation processes of PEG segments of block copolymers as describedin WO96/32434, WO96/33233 and WO97/06202.

Thus formed PEG segment portion and poly (amino acid or a derivativethereof) segment portion can take any form of linkage according to themethod used for preparing the copolymers of the general formula (I),(II), (III) or (IV) and any linker may be used for the linkage so longas the objects of the present invention are achieved.

The preparation method is subject to no special limitation. For example,a method can be used in which a PEG derivative having an amino group atone end is used to synthesize a block copolymer through polymerizationstarting from the amino terminal, of N-carboxylic anhydride (NCA) ofprotective amino acid such as β-benzyl-L-aspartate, Nε-Z-L-lysine or thelike, and then the side chains are converted. In this case, the formedcopolymer has a structure of the general formula (I) or (III) and thelinker L¹ takes a structure derived from the terminal structure of thePEG derivative used, which preferably is —(CH₂)_(b)—NH—, b being aninteger of 1-5.

A copolymer of the present invention can be prepared also by a methodcomprising synthesizing a poly(amino acid or a derivative thereof)segment potion and then linking it with a PEG segment portion. In thatcase, the copolymer may have a same structure as that of the product ofabove-described method, or may have a structure of the general formula(II) or (IV). The linker L² is not critical, but preferably—(CH₂)_(c)—CO—, c being an integer of 1-5.

In the general formula (I), (II), (III) or (IV), R⁵ each independentlystands for hydroxyl, oxybenzyl, —NH—(CH₂)_(a)—X, it being preferred thatmost of R⁵ _(s) (generally at least 85%, preferably at least 95%, inparticular, at least 98%, inter alia, 100%) are —NH—(CH₂)_(a)—X. Again,R⁶ in the general formula (III) or (IV) can each independently stand forhydrogen or a protective group, it being preferred that most of R⁶ _(s)are hydrogen atoms. Here the protective group means those normally usedas protective groups of amino, such as Z group, Boc group, acetyl group,trifluoroacetyl group and the like.

X is subject to no particular limitation so long as the copolymerssatisfy the conditions of the present invention (or meet the objects ofthe present invention). It is selected from residues classified intofive groups: i.e.,

-   -   Group A; bulky amine compound residues having a pKa value not        more than 7.4

-   -   Group B; amine compound residues containing both primary amine        and secondary amine, tertiary amine or quaternary ammonium salt

-   -   Group C; amine compound residues containing primary amine only        —(CH₂)_(f)—NH₂    -   Group D; amine compound residues containing secondary amine,        tertiary amine or quaternary ammonium salt only, which are not        included in Group A        —(NR⁷(CH₂)_(d))_(e)—NHR⁸, —N(CH₃)₂ or —N(CH₂CH₃)₂        and    -   Group E; residues of compounds other than amine

Those copolymers represented by the general formula (I) or (II) maycontain any one residue only selected from the residues of Groups A andB; where they contain a residue of Group C, they must concurrentlycontain at least one residue selected from the residues of Groups A andD; and where they contain a residue of Group D, they must concurrentlycontain at least one residue selected from the residues of Groups B andC. Group E residue or residues can be contained in the copolymers tovary physical properties of the copolymers, but in that case thestructure of the copolymers except the Group E residue portions mustsatisfy the above requirements. The copolymers of the general formula(III) or (IV) may contain any one residue only which is selected fromthe residues of Groups A, B and D, where at least one of R⁶s is hydrogenatom. Requirements for the copolymers containing Group C residue(s) andGroup (E) residue(s) are same as above.

Examples of preferred residues in each Group are shown referring to theformulae. In the formulae, X² in Group A is hydrogen or C₁₋₆ alkyl; inGroup B, X³ is amino C₁₋₆ alkyl, R⁷ is hydrogen or methyl, and d and eare each an integer of 1-5; in Group C, f is an integer of 0-15; inGroup D, d and e each is an integer of 1-5 and R⁸ is a protective groupsuch as Z group, Boc group, acetyl group, trifluoroacetyl group and thelike; and in Group E, g can be an integer of 0-15.

As a method for introducing these residues into side chains of polyaminoacid structure, particularly when the latter is polyaspartic acidstructure, the introduction can be conveniently carried out bytransesterification from ester to amide by aminolysis ofpoly-(β-benzyl-L-aspartate) portion as described in, for example, JP2,777,530. As another method, the benzyl ester is converted topolyaspartic acid or polyglutamic acid by catalystic reduction orhydrolysis using an acid or alkali, and thereafter a compound havingthese residues is linked thereto using a condensing agent or the like.

These cationic side chains may be in the form of salt. In that case, asthe pair ions to form the salt, Cl⁻, Br⁻, I⁻, (½SO₄)⁻, NO₃ ⁻, (½CO₃)⁻,(⅓PO₄)⁻, CH₃COO⁻, CF₃COO⁻, CH₃SO₃ ⁻, CF₃SO₃ ⁻ and the like can be named.

R² in the general formula (I), (II), (III) or (IV) stands for methyleneor ethylene, and where R² is methylene, the copolymer corresponds topoly(aspartic acid derivative), and where R² is ethylene, corresponds topoly(glutamic acid derivative). When R² in these general formulae standsfor both methylene and ethylene groups, the recurring units of asparticacid derivative and glutamic acid derivative may be present formingblocks respectively, or may be present at random.

R³ in the general formula (I) or (III) stands for hydrogen, a protectivegroup, hydrophobic group or polymerizable group. As the protectivegroup, C₁₋₆ alkylcarbonyl, preferably acetyl, can be named. As thehydrophobic group, derivatives of benzene, naphthalene, anthracene,pyrene and the like can be named. As the polymerizable groups,methacryloyl and acryloyl can be named, and when copolymers of thegeneral formula (I) or (III) have such polymerizable groups, they can beused as those which are generally referred to as macromers. For example,after formation of PIC micelles, the copolymers can be crosslinked viathese polymerizable groups, using other comonomer(s) where necessary.

As the means to introduce these protective groups, hydrophobic groups orpolymerizable groups into terminals of the copolymers, those used inordinary syntheses such as method of using acid halide, method of usingacid anhydride, method of using active ester, and the like can be named.

R⁴ in the general formula (II) or (IV) can be hydroxyl, oxybenzyl, or—NH—(CH₂)_(a)—X, similar to R⁵. When the block copolymer is prepared bya method comprising synthesizing a poly(amino acid or a derivativethereof) segment by polymerizing NCA of protective amino acid using alow molecular weight initiator, and then linking it with a PEG segment,R⁴ may take a structure derived from the initiator used, i.e., —NH—R⁹,R⁹ being unsubstituted or substituted straight or branched chain C₁₋₂₀alkyl.

Chain lengths of the PEG segment and poly(amino acid or a derivativethereof) segment are specified by m and n, respectively, m being aninteger of 5-20,000, preferably 10-5,000, inter alia, 40-500, and nbeing 2-5,000, preferably 5-1,000, inter alia, 10-200. However, when thecopolymers of the general formula (I), (II), (III) or (IV) are thoseforming PIC micelles with nucleic acid or anionic proteins, the chainlengths are not limited. Therefore, while the terms, polyethylene glycoland polycation, are used in this specification for convenience, “poly”therein signifies the concept encompassing those customarily classifiedunder “oligo”.

Again, x, y and z which specify the constitution ratio of poly-(aminoacid or a derivative thereof ) segment are, respectively, an integer of0-5,000 (provided it is not greater than n), an integer of 0-4,999, andan integer of 1-4,999 (provided that z is smaller than n and y+z is notgreater than n). Preferably, z lies within a range of 10-n-10. Eachconstituent component may be distributed at random or as blocks.

Those copolymers represented by the general formula (I), (II), (III) or(IV) can conveniently form PIC micelles having polyion complex ofpolycation portion of said copolymer and, for example, nucleic acid asthe core and the PEG layer as the shell, when stirred in an aqueousmedium (which may contain water-miscible organic solvent) at roomtemperature, with nucleic acid, e.g., genes encoding other genes usefulfor known gene therapy or therapeutically necessary proteins; DNAfragments such as plasmids, RNA fragments, antisense DNA and the likewhich contain such genes; or anionic proteins (or peptides) (inparticular, those anionically chargeable at physiological pH). Accordingto the present invention, such PIC or PIC micelles themselves also areprovided.

Hereinafter the invention is more specifically explained, referring tospecific examples, it being understood that the examples are givenexclusively for the sake of explanation.

EXAMPLE 1 Synthesis of Polyethylene Glycol/poly(β-benzyl-L-aspartate)-AcBlock Copolymer

Polyethylene glycol (MeO-PEG-NH₂) with methoxy at one end andaminopropyl at the other end, having an average molecular weight of12,000 was dissolved in methylene chloride, and to which a solution ofβ-benzyl-L-aspartate-N-carboxylic anhydride (BLA-NCA) in a mixed solventof N,N-dimethylformamide (DMF) and methylene chloride was added.Allowing the components to react at 40° C. for two days, polyethyleneglycol-poly(β-benzyl-L-aspartate) block copolymer (MeO-PEG-PBLA) wasobtained. Further the N-terminal was acetylated with acetic anhydride,to provide MeO-PEG-PBLA-Ac. The average molecular weight of the PBLAportion was 14,000 and the degree of polymerization was 68, asdetermined by ¹H-NMR analysis.

EXAMPLE 2 Preparation of Polyethylene Glycol/polycation Block Copolymerby Aminolysis with Morpholinopropylamine

MeO-PEG-PBLA-Ac as obtained in Example 1 was dissolved in benzene andlyophilized. Morpholinopropylamine was distilled under reduced pressurewith calcium hydride serving as a desiccant.

MeO-PEG-PBLA-Ac was dissolved in dry DMF, to which 10(mol) eq. ofmorpholinopropylamine to the PBLA unit was added and stirred for 24hours at 40° C. in argon atmosphere. After the 24 hours, the reactionsolution was added dropwisely into 10% aqueous acetic acid solution,followed by dialysis against 0.01N-aqueous hydrochloric acid solutionwith a dialyzer with MWCO=3,500. Evaporating and lyophilizing the liquidinside of the dialyzer, the object product (MeO-PEG-MOPA) was obtainedas a white solid. The structure of the polymer was confirmed by means of¹H-NMR.

In consequence, the peaks attributable to the benzyl groups in theMeO-PEG-PBLA-Ac completely disappeared and newly proton signalsoriginated from the amide formation were confirmed. From integralvalues, approximately quantitative progress in aminolysis of polymerside chains was confirmed.

EXAMPLE 3 Preparation of Polyethylene Glycol/polycation Block Copolymer

by aminolysis with diethylenetriamine

MeO-PEG-PBLA-Ac as obtained in Example 1 was dissolved in benzene andlyophilized. Diethylenetriamine was distilled under reduced pressurewith calcium hydride serving as a desiccant.

MeO-PEG-PBLA-Ac was dissolved in dry DMF, to which 50 (mol) eq. ofdiethylenetriamine to the PBLA unit was added and stirred for 24 hoursat 40° C. in argon atmosphere. After the 24 hours, the reaction solutionwas added dropwisely into 10% aqueous acetic acid solution, followed bydialysis against 0.01N-aqueous hydrochloric acid solution with adialyzer with MWCO=3,500. Lyophilizing the liquid inside the dialyzer,the object product (MEO-PEG-DET) was obtained as a white solid.

The structure of the produced polymer was confirmed by ¹H-NMR.

In consequence, the peaks attributable to the benzyl groups in theMeO-PEG-PBLA-Ac completely disappeared and newly proton signalsoriginated from the amide formation were confirmed. From integralvalues, approximately quantitative progress in aminolysis of polymerside chains was confirmed.

COMPARATIVE EXAMPLE 1

Preparation of Polyethylene Glycol/polycation Block Copolymer

by aminolysis with N,N-dimethylpropylamine

MeO-PEG-PBLA-Ac as obtained in Example 1 was dissolved in benzene andlyophilized. N,N-dimethylpropylamine was distilled under reducedpressure with calcium hydride serving as a desiccant.

MeO-PEG-PBLA-Ac was dissolved in dry DMF, to which 10 (mol) eq. ofN,N-dimethylpropylamine to the PBLA unit was added and stirred for 24hours at 40° C. in argon atmosphere. After the 24 hours, the reactionsolution was added dropwisely into 10% aqueous acetic acid solution,followed by dialysis against 0.01N-aqueous hydrochloric acid solutionwith a dialyzer with MWCO=3,500. Evaporating and lyophilizing the liquidinside the dialyzer, the object product (MEO-PEG-DMAPA) was obtained asa white solid.

The structure of the produced polymer was confirmed by ¹H-NMR.

In consequence, the peaks attributable to the benzyl groups in theMeO-PEG-PBLA-Ac completely disappeared and newly proton signalsoriginated from the amide formation were confirmed. From integralvalues, approximately quantitative progress in aminolysis of polymerside chains was confirmed.

COMPARATIVE EXAMPLE 2

Preparation of Polyethylene Glycol/polycation Block Copolymer

by aminolysis with diaminopropane

MeO-PEG-PBLA-Ac as obtained in Example 1 was dissolved in benzene andlyophilized. Diaminopropane was distilled under reduced pressure withcalcium hydride serving as a desiccant.

MeO-PEG-PBLA-Ac was dissolved in dry DMF, to which 50 (mol) eq. ofdiaminopropane to the PBLA unit was added and stirred for 24 hours at40° C. in argon atmosphere. After the 24 hours, the reaction solutionwas added dropwisely into 10% aqueous acetic acid solution, followed bydialysis against 0.01N-aqueous hydrochloric acid solution with adialyzer with MWCO=3,500. Evaporating and lyophilizing the liquid insidethe dialyzer, the object product (MEO-PEG-DAP) was obtained as a whitesolid.

The structure of the produced polymer was confirmed by ¹H-NMR.

In consequence, the peaks attributable to the benzyl groups in theMeO-PEG-PBLA-Ac completely disappeared and newly proton signalsoriginated from the amide formation were confirmed. From integralvalues, approximately quantitative progress in aminolysis of polymerside chains was confirmed.

EXAMPLE 4 Formulation of PIC Micelles of Block Copolymer and Plasmid DNA

Plasmid DNA encoding luciferase (p-DNA, pGL3-Luc) was purified with Maxikit of Quiagen GmbH and formulated into a solution at a concentration of50 μg/ml with tris-hydrochloric acid buffer (10 mM, pH=7.4). Solutionsof the copolymers as prepared in Example 2, Example 3, ComparativeExample 1 and Comparative Example 2 (tris-hydrochloric acid buffer (10mM, pH=7.4)) were each blended with the p-DNA solution to satisfy theN/P ratio each. The solutions were let stand an overnight in a darkplace at room temperature. The solutions were transparent and formationof no aggregate or precipitate was observed. Here “N/P ratio” refers to“concentration of cation residues in the tested block copolymer” over“concentration of phosphoric acid group in p-DNA”, while for theMeO-PEG-DET as prepared in Example 3 alone, it was calculated from theconcentration of primary amino group only.

EXAMPLE 5 Characterization by Electrophoresis

A 0.9 wt % agarose gel was prepared and electrophoresis was conducted atp-DNA concentration of 0.17 μg/lane and using as the migration buffertris-hydrochloric acid buffer (3.3 mM, pH=7.4) under the conditions ofapplied voltage of 50V and migration time of 2 hours. Thereafterstaining was conducted with ethidium bromide solution (0.5 mg/L) for anhour.

FIG. 1 shows the result with MeO-PEG-MOPA and p-DNA mixed solution andFIG. 2, the result with MeO-PEG-DET and p-DNA mixed solution. In both,at the N/P ratio>1, the band attributable to free DNA disappeared,suggesting formation of PIC micelles. Also in consideration of the givendefinition of the NT/P ratio, it was suggested that the primary amine inthe block copolymers mainly participated in the formation of theassociated particles with MeO-PEG-DET and secondary amine scarcelyparticipated. In the associated particles the secondary amine remainedin the system in deprotonated state, which could be interpreted asretaining buffer ability.

EXAMPLE 6 Characterization of Dynamic Light Scattering Method (DLS)

The particle diameter measurements were conducted by dynamic lightscattering. The results were as shown in FIGS. 3-6. FIGS. 3 and 4 showthe results with MeO-PEG-MOPA and p-DNA mixed solutions and FIGS. 5 and6, those with MeO-PEG-DET and p-DNA mixed solutions. FIGS. 3 and 5 showthe particle size distribution at N/P ratio of 1, and FIGS. 4 and 6 showthe results of the measurements at various N/P ratios. These resultsdemonstrate formation of single-peak distribution of associatedparticles having the particle sizes ranging 80-100 nanometers in bothsystems, and it can be understood they have approximately constantparticle size irrelevant to the N/P ratio.

EXAMPLE 7 Characterization by Zeta Potential Measurement

The results of zeta potential measurements are shown in FIGS. 7 and 8.FIG. 7 shows the result with MeO-PEG-MOPA and p-DNA mixed solutions andFIG. 8 shows those of MeO-PEG-DET and p-DNA mixed solutions. In bothsystems a minor tendency for positive charging with rise in N/P ratiowas observed but the absolute values were very small, and showed zetapotentials close to 0 mV.

This signifies that surfaces of formed associated particles areelectrically close to neutral, suggesting a core-shell type structurehaving a PEG layer at the surface and polyion complex at the inner core.

EXAMPLE 8 Characterization by Ethidium Bromide Assay

Degree of condensation of p-DNAs inside the PIC micelles was evaluatedby ethidium bromide assay. The fluorescence intensity of 590 nm reflectsthe degree of condensation of p-DNAs inside the micelles. That is,higher fluorescence intensity signifies that p-DNAs are in more relaxed(free) state, and lower fluorescence intensity, more condensed state ofthe p-DNAs. In plotting, the fluorescence intensity of 590 nm measuredupon addition of ethidium bromide to p-DNA at a prescribed concentrationwas standardized as 100, and the measured values were plotted asrelative intensities thereto.

The measured results with the micelles formed of MeO-PEG-MOPA and p-DNAswere as shown in FIG. 9. With rise in the N/P ratio, the fluorescenceintensity showed a tendency for minor decrease, but even at the N/Pratio of 10, the intensity was still about 70. Compared with blockcopolymers which have been studied in the past, this value is very high(where a polycation which effectively condenses DNAs such as polylysineis used, the relative fluorescence intensity decreases to about 5 atN/P>1).

Also the results of similar measurements of a system to which table saltwas added at a physiological concentration (150 mM) were as shown inFIG. 10. The relative fluorescence intensity further approached to 100,indicating that the p-DNAs in the micelles were in a relaxed state atabout the same level with free p-DNAs. In actual administration ofmicelles to cutured cells or living bodies, the state as illustrated inFIG. 10 is considered closer to the real conditions, rather than that asillustrated in FIG. 9.

The results of the measurements of the micelles formed of MeO-PEG-DETand p-DNAs were as shown in FIG. 11. Within the N/P ratio range of 0-2,rapid decrease in the fluorescence intensity was observed with the risein the N/P ratio, and during the NIP ratio increase from 2 to 10, gentledecrease in the fluorescence intensity was observed.

EXAMPLE 9 Evaluation of Gene Transfer Effectivity

A micelle of MeO-PEG-MOPA and p-DNA was prepared at N/P=5, which wascontacted with cultured cells (293 T cells) for a fixed time andthereafter the culture medium was replaced with a new one. Continuingthe incubation for further 24 hours, the gene transfer effectivity wasevaluated by luciferase assay and quantitation of proteins. The resultswere as shown in FIG. 12. When the case of 4 hours' contact of themicelle with the cells is compared with that of 24 hours' contact, itcan be understood that the extension of the contact time drasticallyimproved the gene transfer effectivity. This is presumed to be caused bythe change in release rate of the genes enclosed in the micelle.

As for MeO-PEG-DET, those MeO-PEG-DMAPA and MeO-PEG-DAP as prepared inComparative Examples 1 and 2 were used as the control samples forevaluation. MeO-PEG-DET had both primary amine and secondary amine atthe same time; MeO-PEG-DMAPA had tertiary amine only; and MeO-PEG-DAPhad primary amine only. Micelles of the three kinds of copolymers eachwith p-DNAs were prepared at N/P=10, which were contacted with culturedcells (293 T cells) for 24 hours and the culture medium was renewed.After further 24 hours' incubation, gene expression effectivity wasevaluated by means of luciferase assay and quantitation of proteins. Theresults were as shown in FIG. 13. MeO-PEG-DET exhibited higher genetransfer effectivity than the other two kinds of copolymers. This isconsidered to be caused by the introduction of the amine (primary)participating in the complex formation and the amine (secondary), whichis expected to exhibit buffer ability, into one copolymer, whereby thetransfer of p-DNAs enclosed in the PIC micelles taken into the cells ispromoted by the proton sponge effect.

INDUSTRIAL UTILIZABILITY

According to the present invention, PIC micelles having high genetransfer effectivity can be provided. The present invention also enablesprovision of more useful non-viral gene vectors. Therefore, the presentinvention is useful for medical industries.

1. A polyethylene glycol/polycation block copolymer represented byformula (I) or a salt thereof,

wherein m is an integer of 5-20,000, n is an integer of 2-5,000, x is aninteger of 0-5,000, with the proviso that x is not greater than n, R¹stands for hydrogen, or a substituted or unsubstituted, straight orbranched chain C₁₋₁₂ alkyl, L¹ stands for a linker, R² stands formethylene or ethylene, R³ stands for hydrogen, a protective group, ahydrophobic group, or a polymerizable group, and R⁵s each independentlystand for hydroxyl, oxybenzyl, or —NH—(CH₂)_(a)—X, with the proviso thatat least 85% of R⁵s are —NH—(CH₂)_(a)—X, wherein X independently standsfor one of the following:

wherein a is an integer of 1-5, X² stands for hydrogen or C₁₋₆ alkyl, X³stands for amino C₁₋₆ alkyl, R⁷ stands for hydrogen or methyl, d standsfor an integer of 1-5, e stands for an integer of 1-5, f stands for aninteger of 0-15, and R⁸ stands for Boc, acetyl, or trifluoroacetyl.
 2. Acopolymer as set forth in claim 1, in which R¹ is methyl.
 3. A copolymeras set forth in claim 1, in which R¹ stands for substituted straight orbranched chain C₁₋₁₂ alkyl, wherein the substituent is acetalizedformyl, cyano, formyl, carboxyl, amino, C₁₋₆ alkoxycarbonyl, C₂₋₇acylamide, same or different tri-C₁₋₆ alkylsiloxy, siloxy or silylamino.4. A copolymer as set forth in claim 1, in which L¹ is —(CH₂)_(b)—NH—, bbeing an integer of 1-5.
 5. A copolymer as set forth in claim 1, inwhich R² is methylene.
 6. A copolymer as set forth in claim 1, in whichR³ is acetyl, acryloyl or methacryloyl.
 7. A polyion complex comprisinga copolymer as described in claim 1 and nucleic acid or anionic protein.8. A polyion complex as described in claim 7, which is in the form of apolymer micelle carrying nucleic acid or anionic protein in its coreportion wherein the shell portion comprises a polyethylene glycolsegment.
 9. A polyethylene glycol/polycation block copolymer representedby formula (III) or a salt thereof,

wherein m is an integer of 5-20,000, n is an integer of 2-5,000, y is aninteger of 0-4,999, z is an integer of 1-4,999, with the proviso that zis less than n and y+z is not more than n, R¹ stands for hydrogen, or asubstituted or unsubstituted, straight or branched chain C₁₋₁₂ alkyl, L¹stands for a linker, R² stands for methylene or ethylene, R³ stands forhydrogen, a protective group, a hydrophobic group, or a polymerizablegroup, R⁵s each independently stand for hydroxyl, oxybenzyl, or—NH—(CH₂)_(a)—X, wherein X independently stands for one of thefollowing:

wherein a is an integer of 1-5, X² stands for hydrogen or C₁₋₆ alkyl, X³stands for amino C₁₋₆ alkyl, R⁷ stands for hydrogen or methyl, d standsfor an integer of 1-5, e stands for an integer of 1-5, f stands for aninteger of 0-15, R⁸ stands for Boc, acetyl, or trifluoroacetyl, and R⁶each independently stands for hydrogen, Boc, acetyl, or trifluoroacetyl.